It is desirable to position a telescope in orbit around the earth to obtain clear images of remote targets such as astronomical phenomena. As used herein, the term “telescope” refers to any device that collects, magnifies, or concentrates light and/or electromagnetic radiation received from and/or transmitted to a distant point. When a telescope is positioned in orbit, the light originating from such remote targets can be collected without the distortion that occurs when that light passes through the earth's atmosphere. Accordingly, several telescopes have been placed in orbit around the earth.
Historically, such space telescopes have been mounted on platforms (satellites) whose only purpose has been to serve as a base for mounting such telescopes. Because these platforms serve only this single purpose, they are inertially pointed with respect to the remote target, i.e., these platforms continuously face towards the remote target as they orbit the earth. An inertial platform needs only fine pointing and tracking capabilities to maintain its orientation with respect to the remote target.
Space telescopes typically require routine maintenance. Such maintenance has historically been provided by humans travelling to space for the sole purpose of providing such maintenance. Space travel, however, is expensive. Economic considerations have lead to a reduction in the number of space flights, with further curtailment of spaceflights likely to occur in the future. Accordingly, it is desirable to mount space telescopes on platforms that serve multiple functions so as to reduce or eliminate the need to travel to space on single-purpose maintenance missions.
One solution is to mount a space telescope on an orbiting space station that is continuously populated and/or routinely visited by humans, such as the International Space Station. By using a space station as a mounting platform for a space telescope, the need to conduct single purpose space missions to perform routine maintenance on a space telescope can be eliminated. This is because of the continuous or regular presence of humans on board such a space station who would be available to perform any required maintenance.
Mounting a space telescope on a space station provides an opportunity to increase the amount of light that can be collected. Because of the size of a space station, multiple, spaced-apart, synchronized mirrors can be used to collect light which, in turn, increases the total light collecting area of the space telescope. On earth, telescopes have been designed and constructed that use multiple mirrors that are spaced apart from one another. Each mirror reflects an image in a focused beam to a collector that is positioned to intersect each focused beam and that is designed to combine all the images to form a composite image. Such multi-mirror earth based telescopes are designed and constructed such that the focal length from each mirror to the collector is substantially equal. This ensures that each image reaching the collector is substantially the same size. This, in turn, greatly simplifies the process of combining the multiple images.
Space telescopes having multiple mirrors have not previously been placed in orbit because large platforms capable of supporting multiple mirrors have not been available. A space station would offer a much larger base for mounting mirrors to collect light than any previous orbiting platform. This makes it possible to position multiple collection mirrors around the space station to greatly increase the overall collection area of the telescope. However, because space stations (and/or other large orbiting platforms) are not designed to be telescope platforms, they are not constructed to permit placement of multiple mirrors in a configuration where each mirror has the same focal length to the collector. Thus the image projected by each mirror would be different in size from the image projected by each other mirror when it intersects the collector. Additionally, space stations orbiting the earth (or other planets) are typically nadir pointing (i.e., continuously facing the earth) and, as a result, their orientation with respect to the remote target is continuously changing.